In December 2000, the International Standards Organization (ISO) ratified a new image compression standard known as the JPEG2000 image compression standard, as described in “Information Technology—JPEG2000 Image Coding System, ISO/IEC International Standard 15444-1, ITU Recommendation T.800”. The JPEG2000 standard is based on the EBCOT algorithm as described by Taubman (David Taubman, “High performance scalable compression with EBCOT,” IEEE Transactions on Image Processing, 9(7), pp. 1158-1170, July 2000). The JPEG2000 standard represents a paradigm shift from the old JPEG standard. With the explosion of devices such as PDAs and cellphones, it is anticipated that a high-resolution JPEG2000 compressed image stored at an image server may be accessed by a variety of clients with differing needs for resolution and image quality. One of the key properties of the JPEG2000 standard is that it is possible to parse a JPEG2000 bit-stream to extract a lower resolution and/or quality image without having to perform dequantization and requantization. In some cases, even arithmetic decoding may not be required.
Transcoding refers to converting a JPEG2000 compressed image to another JPEG2000 compressed image, typically at a lower bit-rate and/or resolution. Currently, the commercially available “Kakadu software”, developed by David Taubman and published at Internet: URL:
http://www.kakadusoftware.com, provides rudimentary transcoding capability. The JPEG2000 bit-stream can be configured to have multiple quality layers, where each additional layer improves the quality of the compressed image. The performance of the transcoder in Kakadu is very poor if it is used to transcode an existing JPEG2000 bit-stream to a rate that is very far from the nearest bit-rate layer boundary. In co-pending, commonly-assigned U.S. patent application Ser. No. 10/108,151, entitled “Producing and Encoding Rate-distortion Information Allowing Optimal Transcoding of Compressed Digital Image” and filed Mar. 3, 2002, Joshi and Deever proposed a method to perform optimal transcoding, but their method requires that the JPEG2000 encoder that generated the initial JPEG2000 compressed image also stored rate-distortion information to facilitate transcoding.
In practice, it may be necessary to transcode a JPEG2000 compressed image for which no rate-distortion information has been stored. In such cases, it is desirable that the quality of the image transcoded to a lower bit-rate be as close as possible to a JPEG2000 image compressed directly to the lower bit-rate.